The invention relates in general to acceleration sensors and in particular to MEMS type acceleration sensors.
Acceleration sensors and switches are used in various commercial and military applications, for example, projectiles that carry explosives. Acceleration sensors that close an electrical circuit upon impact of a projectile are often called impact sensors or G-switches (Gravity switches) or PD (point detonating) switches. Micro-electro mechanical systems (MEMS) are used in the fuzes and safe and arm systems of some projectiles. MEMS devices are small, lightweight, accurate, reliable and often cheaper than conventional devices. The MEMS G-switches shown in U.S. Pat. Nos. 6,737,979 and 6,619,123 move only in the plane of the substrate and are not able to filter out large cross axis accelerations that may be induced by spin.
U.S. Pat. Nos. 6,765,160 and 7,038,150 show MEMS type acceleration switches designed for movement out of the plane of the substrate. As these devices use folded springs to support the mass, the mass is also able to move in the plane of the substrate, which can lead to binding of the mass during spinning of the projectile. In addition, the mass has a relatively large surface area that contacts the contact pad, resulting in high contact resistance and switch bounce. Furthermore, the device in U.S. Pat. No. 7,038,150 uses a mass that is 8 to 300 times thicker than the springs, thereby complicating the manufacturing process.